If the problem isn't Quark's main program files, you could try a couple of other smaller things that may help.
Close Quark. Go to the /Applications/QuarkXPress 6.0/ folder. Open the folder named "jaws" and empty it contents to the Trash. Those are just Quark's own font cache files. Try launching Quark again. If it doesn't work, try this next.
Once again, close Quark. Open both the "XTension" and "XTension Disabled" folders. Move all items in "XTension" to "XTension Disabled". Launch Quark. If it works then, you have a corrupt extension.
If that still doesn't help, make sure Quark is up to date. You list that you're using 6.1. Quark made a major free update to Quark 6 as version 6.5.2. Download and install the updates. You first need to download and install the
6.5 update. Then download and install the
6.5.2 update.
If it still doesn't work after that, then reinstall Quark from the original disk and apply all updates.